One example of typical non-volatile memories is a resistive random access memory (ReRAM) that uses resistance change caused by voltage application to insulating films provided between top and bottom electrodes. Researches and developments have been conducted vigorously to achieve high performance, highly reliable, and low cost ReRAMs.
ReRAMs are formed in a simple metal-insulator-metal (MIM) structure and have a low process temperature, making ReRAMs be compatible with complementary metal-oxide-semiconductor (CMOS) logic. Two main ways of memory operations of ReRAMs have been proposed as follows. One is a mechanism for forming conductive paths (filaments) extending between top and bottom electrodes by applying a voltage to diffuse an electrode material into an insulating film. Copper (Cu) and platinum (Pt) have been examined as typical electrode materials. The other is a mechanism for forming filaments between top and bottom electrodes by introducing oxygen vacancy into the insulating film by reduction reaction, using an ionic metal oxide as the insulating film. As a typical ionic metal oxide, nickel oxide (NiO), hafnium oxide (HfO2), titanium oxide (TiO2), and tantalum oxide (TaOx) have been examined. Memory operation is carried out using a low resistance state in which filaments are connected and a high resistance state in which filaments are disconnected in accordance with changes of the direction of voltage application (see, e.g., NON-PATENT LITERATURE 1, 2).
ReRAMs require initial operation called forming, which is regarded as soft breakdown, to form the filaments. To decrease a high forming voltage as a result of random forming of the filaments, it has been disclosed that a polycrystalline oxide layer, which acts as an insulating film, and an amorphous layer thicker than the polycrystalline oxide layer are stacked to use filaments generated in the boundaries of the polycrystalline oxide layer (see, e.g., PATENT LITERATURE 1). An oxygen drawing substance is distributed at an interface between the electrode and the insulating film to introduce oxygen vacancy into the insulating layer (see, e.g., PATENT LITERATURE 2).
However, in the formation of the filaments generating from the boundaries between polycrystalline oxide layers disclosed in the above PATENT LITERATURE, the number and physical property of the boundaries are largely affected by the fabrication conditions of the polycrystalline layers, causing variations in the forming voltage. Meanwhile, the oxygen depleting substance to be distributed should be an oxide with oxygen vacancy, as represented by TiOx. It is difficult to regulate the amount of oxygen vacancy that determines the oxygen drawing amount from the insulating film. This makes it difficult to control the formation of filaments.
In one report, the oxygen vacancy is introduced by removing oxygen from the top surface of single crystal SrTiO3. Oxygen of O—Ti bonds on the top surface of SrTiO3 is reacted with trimethylaluminium (TMA), Al(CH3)3, which is a raw material for film forming of an aluminum oxide (Al2O3) film by atomic layer deposition (ALD). As a result, a two-dimensional electron gas is generated (see, e.g., NON-PATENT LITERATURE 3). However, the removal of oxygen from the top surface suggested by the above NON-PATENT LITERATURE is limited to O—Ti bonds ordered on the single crystal substrate. A reaction between TMA and oxygen of O—Ti bonds in titanium oxide (TiO2) formed by amorphous body or polycrystal and the corresponding introduction of oxygen vacancy have not been suggested.
In another report, natural oxide films can be removed by reacting oxygen of a natural oxide generated on a GaAs substrate (e.g., GaOx, AsOx) and TMA in the process of generating an aluminum oxide (Al2O3) film by ALD with TMA being as a raw material (see, e.g., NON-PATENT LITERATURE 4). However, the natural oxide disclosed in the above NON-PATENT LITERATURE is an oxygen-poor oxide, according to its stoichiometric ratio, in which oxygen easily reacts with TMA and no suggestion has been made about introducing oxygen vacancy by the reaction of oxygen in an oxygen-rich oxide having at least a stoichiometric composition ratio.